Ch. II.] FORMS OF STRATIFICATION. 13 



bleau, used for pavement in France. More commonly we find sand and 

 clay, or clay and marl, intermixed in tne same mass. When the sand 

 and clay are each in considerable quantity, the mixture is called loam. 

 If there is much calcareous matter in clay it is called marl ; but this 

 term has unfortunately been used so vaguely, as often to be very ambig- 

 uous. It has been applied to substances in which there is no lime ; as, 

 to that red loam usually called red marl in certain parts of England. 

 Agriculturists were in the habit of calling any soil a marl, which, like 

 true marl, fell to pieces readily on exposure to the air. Hence arose the 

 confusion of using this name for soils which, consisting of loam, were 

 easily worked with the plough, though devoid of lime. 



Marl slate bears the same relation to marl which shale bears to clay> 

 being a calcareous shale. It is very abundant in some countries, as in 

 the Swiss Alps. Argillaceous or marly limestone is also of common oc- 

 currence. 



There are few other kinds of rock which enter so largely into the 

 composition of sedimentary strata as to make it necessary to dwell here 

 on their characters. I may, however, mention two others, — magnesian 

 limestone or dolomite, and gypsum. Magnesian limestone is composed 

 of carbonate of lime and carbonate of magnesia ; the proportion of the 

 latter amounting in some cases to nearly one-half. It effervesces much 

 more slowly and feebly with acids than common limestone. In England 

 this rock is generally of a yellowish color ; but it varies greatly in min- 

 eralogical character, passing from an earthy state to a white compact 

 stone of great hardness. Dolomite, so common in many parts of Ger- 

 many and France, is also a variety of magnesian limestone, usually of a 

 granular texture. 



Gypsum. — Gypsum is a rock composed of sulphuric acid, lime, and 

 water. It is usually a soft whitish-yellow rock, with a texture resembling 

 that of loaf-sugar, but sometimes it is entirely composed of lenticular 

 crystals. It is insoluble in acids, and does not effervesce like chalk and 

 dolomite, because it does not contain carbonic acid gas, or fixed air, the 

 lime being already combined with sulphuric acid, for which it has a 

 stronger affinity than for any other. Anhydrous gypsum is a rare vari- 

 ety, into which water does not enter as a component part. Gypseous 

 marl is a mixture of gypsum and marl. Alabaster is a granular and 

 compact variety of gypsum found in masses large enough to be used in 

 sculpture and architecture. It is sometimes a pure snow-white substance, 

 as that of Volterra in Tuscany, well known as being carved for works of 

 art in Florence and Leghorn. It is a softer stone than marble, and more 

 easily wrought. 



Forms of stratification. — A series of strata sometimes consists of one 

 of the above rocks, sometimes of two or more in alternating beds. 

 Thus, in the coal districts of England, for example, we often pass through 

 several beds of sandstone, some of finer, others of coarser grain, some 

 white, others of a dark color, and below these, layers of shale and sand- 

 stone or beds of shale, divisible into leaf-like laminae, and containing 



